Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses. Examples of fluoroelastomers include copolymers comprising units of vinylidene fluoride (VF.sub.2) and units of at least one other copolymerizable fluorine-containing major monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether).
In order to develop the physical properties necessary for some end use applications, fluoroelastomers must be crosslinked. Typical curatives for promoting crosslinking include polyamines, polyols and the combination of an organic peroxide and a multifunctional unsaturated coagent. All these compounds form crosslinks by reacting with a cure site on the fluoroelastomer polymer chain. Examples of cure sites include a double bond, or a labile hydrogen, bromine, iodine, or chlorine atom. A common method of introducing a cure site into a fluoroelastomer made by continuous emulsion polymerization is to continuously add a minor amount of a copolymerizable cure site monomer, along with the major monomers (e.g. VF.sub.2, HFP, TFE, PAVE, etc.) to the polymerization reactor. In this manner, cure sites are randomly distributed along the resulting fluoroelastomer polymer chain. Suitable cure site monomers include bromine- or iodine-containing olefins, and bromine- or iodine-containing unsaturated ethers, non-conjugated dienes and 2-hydropentafluoropropylene (2-HPFP). Alternatively, or in addition to cure site monomers, cure sites may be introduced into the fluoroelastomer by conducting the polymerization in the presence of a chain transfer agent containing iodine, bromine or both. In this manner, a bromine or iodine atom is attached to the resulting fluoroelastomer polymer chain at one or both ends. Such chain transfer agents typically have the formula RI.sub.n, RBr.sub.n or RBrI, where R may be a C.sub.1 -C.sub.3 hydrocarbon, a C.sub.1 -C.sub.6 fluorohydrocarbon or chlorofluorohydrocarbon, or a C.sub.2 -C.sub.8 perfluorocarbon, and n is 1 or 2.
Production of such fluoroelastomers by emulsion and solution polymerization methods is well known in the art; see for example U.S. Pat. No. 4,214,060. Generally, fluoroelastomers are produced in an emulsion polymerization process wherein a water-soluble polymerization initiator and a relatively large amount of surfactant are employed. The resulting fluoroelastomer leaves the reactor in the form of a latex which must be degassed (i.e. freed from unreacted monomers), coagulated, filtered and washed. Emulsion processes suffer from several disadvantages including production of polymers having high Mooney viscosity, which tends to make it difficult to process these materials (i.e. mixing, extruding, molding) into cured articles, due to the presence of ionic end groups on the fluoroelastomer polymer chains. Another disadvantage is that the polymer products contain impurities from retained surfactants, coagulants, buffers and defoamers.
On the other hand, in a suspension polymerization process, polymerization is carried out by dispersing one or more monomers, or an organic solvent with monomer dissolved therein, in water and using an oil-soluble organic peroxide. No surfactant or buffer is required and fluoroelastomer is produced in the form of polymer particles which may be directly filtered, i.e. without the need for coagulation, and then washed, thus producing a cleaner polymer than that resulting from an emulsion process. Also, the fluoroelastomer polymer chains are substantially free of ionic end groups so that the Mooney viscosity is relatively low and the polymer has improved processability compared to polymer produced by an emulsion process (U.S. Pat. Nos. 3,801,552, 4,985,520 and 5,824,755).
A disadvantage of suspension polymerization processes disclosed in the prior art is that it is difficult to incorporate a cure site monomer uniformly into the polymer because polymerization rate and polymer molecular weight increase throughout the reaction period. Many cure site monomers, if present in excess, greatly hinder the polymerization reaction, so that the desired polymerization rate and polymer molecular weight can not be attained in suspension polymerization processes of the prior art.